Koichi Hirose is 17 years old during the events of JoJo's Bizarre Adventure: Golden Wind (Part 5).
Understanding Koichi's Age Progression
Koichi's age can be deduced by examining the timeline between Diamond is Unbreakable (Part 4) and Golden Wind (Part 5).
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Part 4: Diamond is Unbreakable (1999)
- During his introduction in Morioh, Koichi Hirose is established as a first-year high school student, the same year as Josuke Higashikata. Based on typical Japanese school age, a first-year high school student is generally 15 years old. Throughout the events of Part 4, which spans a relatively short period, Koichi remains 15.
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Part 5: Golden Wind (2001)
- Golden Wind takes place two years after the events of Diamond is Unbreakable. This two-year time jump directly impacts the ages of returning characters like Koichi.
Given this progression:
| Event | Year | Koichi's Age | Details |
|---|---|---|---|
| Part 4 Begins | 1999 | 15 | First-year high school student in Morioh |
| Part 5 Begins | 2001 | 17 | Two years after Part 4 events |
Therefore, Koichi's appearance in Italy at the beginning of Golden Wind, tasked by Jotaro Kujo to investigate Giorno Giovanna, places him at 17 years of age. His age reflects the passage of time and his continued growth both physically and as a Stand user.
